ROLL C. Ji, Ed.
Internet-Draft Alexander TEI of Thessaloniki
Intended status: Standards Track R. Koutsiamanis
Expires: April 22, 2019 G. Papadopoulos
IMT Atlantique
D. Dujovne
Universidad Diego Portales
N. Montavont
IMT Atlantique
October 19, 2018
Traffic-aware Objective Function
draft-ji-roll-traffic-aware-objective-function-03
Abstract
This document proposes a remaining throughput metric for parent and
DODAG selection. This metric represents the amount of remaining
traffic handling capacity that the node has. This document also
proposes an Objective Function (OF) which uses the proposed metric
for parent and DODAG selection to balance the amount of traffic
between nodes and DODAGs.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 22, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DODAG construction in RPL . . . . . . . . . . . . . . . . . . 3
4. Load distribution problem in RPL . . . . . . . . . . . . . . 3
4.1. Parent selection problem . . . . . . . . . . . . . . . . 4
4.2. DODAG selection problem . . . . . . . . . . . . . . . . . 6
5. TAOF description . . . . . . . . . . . . . . . . . . . . . . 8
6. DIO Metric Container Type extension . . . . . . . . . . . . . 9
7. Enrollment . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Informative references . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
RPL [RFC6550] is an IPv6 Routing protocol for LLNs. It uses
Objective Functions (OF) to construct the Destination Oriented
Directed Acyclic Graph (DODAG) containing the nodes of the network.
The existing OFs defined are OF Zero (OF0) [RFC6552] and Minimum Rank
with Hysteresis OF (MRHOF) [RFC6719]. These OFs specify how nodes in
a DODAG select their preferred parent using different metrics.
The metrics can be separated into two different types, link metrics
(e.g. ETX) and node metrics (e.g. energy). Experimental results
[I-D.qasem-roll-rpl-load-balancing] conclude that using the current
OFs leads to an unbalanced network within which some nodes are
overloaded. Here, a node is overloaded in the sense that it forwards
many more packets than it otherwise would if the network were
balanced. This problem has consequences for the lifetime of the
network because overloaded nodes drain quicker than others, a problem
which becomes even more significant when the overloaded nodes are
near the DODAG root [I-D.qasem-roll-rpl-load-balancing].
Similarly, one DODAG might be overloaded in the same sense compared
to another DODAG, and this will lead to the same consequences for the
whole DODAG as for a specific node.
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This problem is still an open issue. This draft proposes a new way
of parent and DODAG selection as an attempt towards a solution. This
draft proposes a new OF that considers the remaining throughput as a
representation of the remaining traffic handling capacity each node
possesses and which uses this information to balance the amount of
traffic between nodes and DODAGs.
In brief, each node tracks its remaining throughput and appends this
information as a DAG Metric Container option to DIO messages it
sends. When the DIO message is received by child nodes or potential
child nodes, the remaining throughput information is stored and used
to influence the result when RPL parent or DODAG selection is
performed.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. DODAG construction in RPL
RPL uses OFs to construct a DODAG. OFs define the way the nodes
select their preferred parent and DODAG and how they compute the new
rank. A node's rank is always larger than its parent's rank because
the calculation of rank is based on an increment to the parent's
rank. This increment differs for each OF but all OFs include the
MinHopRankIncrease, which is the minimum increase in rank between a
node and a node's parent and a step. Different OFs use different
metrics or constraints to select the preferred parent and DODAG and
to define the step, depending on application requirements. Nodes
obtain these values from DODAG Information Object (DIO) control
messages sent by their neighbor nodes.
The construction of a DODAG starts when the root node sends DIO
messages to its neighbors. After receiving the DIO, these neighbor
nodes select the root as their preferred parent if they wish to join
the DODAG. To announce that they joined the DODAG as its child node,
they send a Destination Advertisement Object (DAO) to their preferred
parent - the DODAG root. After joining the DODAG, these nodes send
their own DIO messages with the new computed rank to their neighbors.
This procedure repeats for every node which joins the DODAG.
4. Load distribution problem in RPL
According to the experiments conducted using existing OFs RPL faces a
load distribution problem in large LLNs. With RPL using existing
OFs, such as MRHOF, an unbalanced network is formed with some nodes
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overloaded and other nodes at rest. This problem is severe for
network performance because overloaded nodes will use up their
available energy faster than other nodes. This is exacerbated for
nodes near the root (within 1 hop distance) or nodes which are the
only parent candidate for other nodes. Additionally, when the
overloaded node shuts down, a big part of the network will become
disconnected and will have to be transferred to another parent or
DODAG. There is a high probability that the children nodes will also
select the same new node as their parent or the same DODAG, leading
to another overloaded node/DODAG. Also, when a node has selected its
parent, it will change only when the parent node is not reachable
(due to battery depletion or packet losses).
The existing OFs usually use a single metric to compare parent
candidates, for example, as described in [RFC6719] the default metric
used in MRHOF is ETX [RFC6551], which represents the number of
transmissions a node expects to make to a destination to successfully
deliver one packet. The result from using a single metric is that
nodes prefer to select the same node as their parent, which according
to [I-D.qasem-roll-rpl-load-balancing] leads to an unbalanced network
with overloaded nodes (node load is indicated by a node's child
count). But the child count does not accurately indicate the load
because among the child nodes some may have higher traffic load and
others may have lower.
The network traffic can be quantified by tracking the packets a node
generates/sends/receives and the amount of energy it consumes.
Energy consumption is strongly correlated to the amount of network
traffic handled by a node since the energy consumption for the
operation of the radio is the primary energy consumer in typical
nodes. However, directly measuring the remaining throughput is both
more accurate and also works when nodes have atypical energy
consumption profiles (e.g. increased node processing or high energy
consumption sensors).
Calculating the remaining throughput then requires knowledge of the
total throughput supported by a node and subtraction of the used
throughput.
4.1. Parent selection problem
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T:4p/s | T:4p/s
U:4p/s (R) | U:4p/s (R)
^ | ^
| | |
+--------------+ | +------+-------+
| | | | |
T:2p/s + T:2p/s + | T:2p/s + T:2p/s +
U:3p/s (A) U:1p/s (B) | U:2p/s (A) U:2p/s (B)
^ ^ | ^ ^
| | | | |
+-----+------+ | | +-----+ +------+
| | | | | | | | |
+ + + + | + + + +
(C1) (C2) (C3) (D1) | (C1) (C2) (C3) (D1)
U:1p/s U:1p/s U:1p/s U:1p/s | U:1p/s U:1p/s U:1p/s U:1p/s
|
Unbalanced | Balanced
Figure 1: Use of Remaining Throughput with nodes with the same
requirements
As a first simple example, an unbalanced network with nodes which all
use the same throughput ("U:") is shown in Figure 1. Nodes A and B
have the same total throughput ("T:"), but node A is overloaded due
to trying to handle more than its ability while node B has a spare
throughput of 2-1=1p/s. Its transformation into a balanced network
is shown on the right and it involves a node (C3) switching parents
from A to B so that the capacity of its parent is no longer exceeded.
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T:6p/s | T:6p/s
U:6p/s (R) | U:6p/s (R)
^ | ^
| | |
+------+-------+ | +--------------+
| | | | |
T:3p/s + T:3p/s + | T:3p/s + T:3p/s +
U:2p/s (A) U:4p/s (B) | U:3p/s (A) U:3p/s (B)
^ ^ | ^ ^
| | | | |
+-----+ +------+ | +-----+------+ |
| | | | | | | | |
+ + + + | + + + +
(C1) (C2) (D1) (D2) | (C1) (C2) (D1) (D2)
U:1p/s U:1p/s U:1p/s U:3p/s | U:1p/s U:1p/s U:1p/s U:3p/s
|
Unbalanced | Balanced
Figure 2: Use of Remaining Throughput with nodes with different
requirements
As a second simple example, an unbalanced network with nodes which
have different throughput ("U:") is shown in Figure 2. In this case,
node B is overloaded and node D1 should move to parent A, which as a
space throughput of 1p/s. Its transformation into a balanced
equivalent network is shown on the right.
4.2. DODAG selection problem
The purpose of the following example is to show the problem of DODAG
selection, and not to focus on selecting the best parent.
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.... DODAG 1 ... ... DODAG 2 .... | .... DODAG 1 ... ... DODAG 2 ....
. . . | . . .
. T:4p/s . T:4p/s . | . T:4p/s . T:4p/s .
. U:4p/s (R1) . U:3p/s (R2) . | . U:5p/s (R1) . U:3p/s (R2) .
. ^ . ^ . | . ^ . ^ .
. | . | . | . | . | .
. +-----+ . +-----+ . | . +-----+ . +-----+ .
. | | . | | . | . | | . | | .
. + + . + + . | . + + . + + .
. (A1) (B1) . (A2) (B2) . | . (A1) (B1) . (A2) (B2) .
. U:3p/s U:1p/s . U:2p/s U:1p/s . | . U:3p/s U:1p/s . U:2p/s U:1p/s .
................ ................ | ................ ................
| ^
| |
^ | +----+
| | |
+ | +
(C) | (C)
U:1p/s | U:1p/s
|
Joining | Joined - Unbalanced
Figure 3: DODAG selection example leading to unbalanced traffic with
RT metric
In the example in Figure 3, there are two DODAGs (DODAG 1 and DODAG
2) that belong to the same RPL Instance and a node (C) that must
select a DODAG. Node C has to pick from the information provided by
its two reachable neighbors: B1 and A2. On the left, node C is shown
before selecting the preferred DODAG, while on the right it is shown
after the DODAG selection.
Node C might choose B1 in DODAG 1 to be its preferred parent since
the traffic information of the two DODAGs is not available. However,
at the root node (R1), it can be observed that the total network
traffic is higher in DODAG 1 and that after node C joins it, the
traffic handling capacity of the root R1 has been exceeded.
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.... DODAG 1 ... ... DODAG 2 .... | .... DODAG 1 ... ... DODAG 2 ....
. . . | . . .
. T:4p/s . T:4p/s . | . T:4p/s . T:4p/s .
. U:4p/s (R1) . U:3p/s (R2) . | . U:4p/s (R1) . U:4p/s (R2) .
. ^ . ^ . | . ^ . ^ .
. | . | . | . | . | .
. +-----+ . +-----+ . | . +-----+ . +-----+ .
. | | . | | . | . | | . | | .
. + + . + + . | . + + . + + .
. (A1) (B1) . (A2) (B2) . | . (A1) (B1) . (A2) (B2) .
. U:3p/s U:1p/s . U:2p/s U:1p/s . | . U:3p/s U:1p/s . U:2p/s U:1p/s .
................ ................ | ................ ................
| ^
| |
^ | +----+
| | |
+ | +
(C) | (C)
U:1p/s | U:1p/s
|
Joining | Joined - Balanced
Figure 4: DODAG selection example leading to balanced traffic with RT
metric
If the traffic handling capacity information is available, then node
C could make a more efficient decision by using DODAG 2 and selecting
node A2 as the preferred parent, as shown in Figure 4. Such a
selection is based on the traffic of the entire DODAG and would not
lead to exceeding the traffic handling capacity of the root R2 since
this root had spare capacity.
5. TAOF description
In this specification, a metric is proposed to be used in the parent
and DODAG selection mechanism, the Remaining Throughput (RT) which
represents the number of packets each node can transmit (send or
forward) during a certain time period. The period used, named
THROUGHPUT_PERIOD, is a parameter common to the whole RPL instance.
This parameter CAN be pre-configured on all the nodes. The period
used SHOULD coincide with a sliding window of the same size used to
calculate the packets transferred during this period to facilitate
the calculation of the remaining possible packet transmissions.
Therefore, whenever the RT value is reported it will refer to the
previous THROUGHPUT_PERIOD period of time. This information is added
in DIO messages and is broadcast to every neighbor.
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At first, each node MUST identify from their neighbor set which nodes
are acceptable to be selected as a parent. For this purpose, the
metric ETX is used as a filter to filter out parent candidates with
low link quality with a preference for nodes with link quality below
a given threshold. The ETX threshold SHOULD be different depending
on application requirements. The suggested value for the relevant
threshold MAX_PATH_COST from MRHOF [RFC6719] is 32768, which means
the specific path has expected transmission counts greater than 256.
When the ETX value is used as a filter, nodes with bad link quality
will not be included in the parent set. This ensures that undue
retransmissions caused by bad links will be avoided. After all the
filtering is done, if any, the node chooses the parent candidate or
DODAG with the highest remaining throughput.
For the purpose of DODAG specifically, the A field in the Routing
Metric/Constraint Flag field object [RFC6551] SHOULD be set to 1,
indicating that the value reported is a maximum. Furthermore, when a
node is calculating the value of RT to broadcast in a DIO, the value
reported SHOULD be the minimum of two values: its parent RT and the
node's own calculated remaining throughput. Thus, the value
broadcasted will be the available remaining throughput in the whole
path from the node to the DODAG root.
This proposal is expected to increase the frequency of parent changes
because the remaining throughput is more likely to be different
between DIO messages, even for DIO messages from the same node.
There are multiple ways to minimize the frequency of unnecessary
parent changes:
a. Use the remaining throughput in combination with another metric
(e.g. child count, hop counts).
b. Use a threshold when comparing the remaining throughput, similar
to the approach in MRHOF [RFC6719]. Switch parents when the
difference of remaining throughput between the original parent
and the alternative parent is above a threshold. This threshold
depends on different factors (e.g. network size, average traffic
load) and SHOULD be defined differently for each use case.
6. DIO Metric Container Type extension
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RT | RT: Remaining Throughput
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: DAG metric container type format.
A DIO message carries fields as described in RFC6550 [RFC6550] and
the available options for the DAG metric container are described in
RFC6551 [RFC6551]. In this specification, a metric container option
is proposed and the detailed format is shown in Figure 5. The
information carried is the RT, represented as a 2 byte unsigned
integer and the unit is packets per THROUGHPUT_PERIOD time.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |Version Number | Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|0| MOP | Prf | DTSN | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAGMC Type (2)| DAGMC Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
// DAG Metric Container data //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Example DIO Message with a DAG Metric Container option
The structure of the DIO Control Message when a DAG Metric Container
option is included is shown in Figure 6. The DAG Metric Container
option type (DAGMC Type in Figure 6) has the value 0x02 as per the
IANA registry for the RPL Control Message Options and is defined in
[RFC6550]. The DAG Metric Container option length (DAGMC Length in
Figure 6) expresses the DAG Metric Container length in bytes. DAG
Metric Container data holds the actual data and is shown further
expanded in Figure 7.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Routing-MC-Type|Res Flags|P|C|O|R| A | Prec | Length (bytes)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: DAG Metric Container (MC) data with Remaining Throughput
(RT) object body
An example DAG Metric Container containing the proposed Metric
Container object is shown in Figure 7. The explicit definition of
the fields is:
Routing-MC-Type: TBD1. The type of the proposed DAGMC extension.
To be assigned by IANA.
Remaining Throughput (RT): The remaining throughput, represented as
a 2-byte unsigned integer in units of packets per
THROUGHPUT_PERIOD time.
7. Enrollment
The RT metric SHOULD be used not only for ongoing parent selection
but also especially within enrollment, i.e. the process a node
follows to join a 6TiSCH network. In accordance to
[I-D.ietf-6tisch-enrollment-enhanced-beacon], the value in RT SHOULD
be used to affect the enrollment process so that a new node will be
able to directly select a DODAG which will be able to cover its
traffic needs and to spread the traffic load between different
DODAGs. More specifically, the pan priority field described in
[I-D.ietf-6tisch-enrollment-enhanced-beacon] can be derived from the
RT value. For this purpose, the RT value SHOULD be used with the
maximum value aggregation mode (A field in the Routing Metric/
Constraint Flag field object [RFC6551] set to 1), to report the
maximum remaining throughput in the whole path to the DODAG root.
The pan priority field is an unsigned 8-bit integer with lower values
signifying higher priority while the RT value is a 16-bit unsigned
integer with higher values signifying more remaining throughput. To
convert the RT value to a pan priority the following formula should
be used:
pan priority = 16 - FLOOR(LOG2(RT + 1))
where LOG2 is the logarithm function with a base of 2. The use of
the LOG function allows having higher accuracy in the low values of
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the remaining throughput, where small value differences are
significant, and lower accuracy in the high values of the remaining
throughput, where small differences are less significant. The
addition of 1 to the RT allows converting RT=0.
8. Security Considerations
The structure of the DIO control message is extended, within the pre-
defined DIO options. Therefore, the security mechanisms defined in
RPL [RFC6550] apply to this proposed extension.
9. IANA Considerations
This proposal requests the allocation of a new value TBD1 for the
metric type "RT" in the Routing-MC-Type field in the DAG MC from
IANA.
Additionally, an Objective Code Point (OCP) with value TBD2 for TAOF
needs to be assigned in the Objective Code Point Registry as
described in Section 20.5 of [RFC6550].
10. Informative references
[I-D.ietf-6tisch-enrollment-enhanced-beacon]
Dujovne, D. and M. Richardson, "IEEE802.15.4 Informational
Element encapsulation of 6tisch Join and Enrollment
Information", draft-ietf-6tisch-enrollment-enhanced-
beacon-00 (work in progress), July 2018.
[I-D.qasem-roll-rpl-load-balancing]
Qasem, M., Al-Dubai, A., Romdhani, I., Ghaleb, B., Hou,
J., and R. Jadhav, "Load Balancing Objective Function in
RPL", draft-qasem-roll-rpl-load-balancing-02 (work in
progress), October 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
.
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[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, DOI 10.17487/RFC6552, March 2012,
.
[RFC6719] Gnawali, O. and P. Levis, "The Minimum Rank with
Hysteresis Objective Function", RFC 6719,
DOI 10.17487/RFC6719, September 2012,
.
Authors' Addresses
Chenyang Ji (editor)
Alexander TEI of Thessaloniki
Department of Informatics
Thessaloniki 57400
GREECE
Email: jichenyang920@gmail.com
Remous-Aris Koutsiamanis
IMT Atlantique
Office B00 - 126A
2 Rue de la Chataigneraie
Cesson-Sevigne - Rennes 35510
FRANCE
Phone: +33 299 12 70 49
Email: aris@ariskou.com
Georgios Papadopoulos
IMT Atlantique
Office B00 - 114A
2 Rue de la Chataigneraie
Cesson-Sevigne - Rennes 35510
FRANCE
Phone: +33 299 12 70 04
Email: georgios.papadopoulos@imt-atlantique.fr
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Diego Dujovne
Universidad Diego Portales
Escuela de Informatica y Telecomunicaciones, Av. Ejercito 441
Santiago, Region Metropolitana
Chile
Phone: +56 (2) 676-8121
Email: diego.dujovne@mail.udp.cl
Nicolas Montavont
IMT Atlantique
Office B00 - 106A
2 Rue de la Chataigneraie
Cesson-Sevigne - Rennes 35510
FRANCE
Phone: +33 299 12 70 23
Email: nicolas.montavont@imt-atlantique.fr
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